Taking as an example the structure of a general solid polymer type fuel cell, the electrode structure is basically one comprised of a polymer electrolyte film sandwiched on one side by a catalyst layer forming a cathode and on the other side by one forming an anode both bonded to the same. Further, these are sandwiched at their two sides by hydrophobic carbon paper etc. in contact with them as gas diffusion layers.
To take out current from a fuel cell of this basic structure, the cathode side is supplied with oxygen or air or another oxidizing gas while the anode side is supplied with hydrogen or another reducing gas from the outside through the gas diffusion layers. For example, when utilizing hydrogen gas and oxygen gas, the energy difference between the chemical reaction H2→2H++2e− (E0=0V) occurring on the catalyst of the anode and the chemical reaction O2+4H++4e−→2H2O (E0=1.23V) occurring on the catalyst of the cathode is utilized to take out current.
For this reason, gas diffusion paths able to supply oxygen gas or hydrogen gas to the catalysts inside the catalyst layers and proton conduction paths and electron conduction paths able to conduct protons and electrons generated on the anode catalyst to the catalyst of the cathode have to run without breakage through at least the catalyst layer or else current cannot be taken out.
As the catalyst suitable for this purpose, a catalyst of a structure using a carbon material, which is high in chemical stability and a good electron conductor, as carrier and carrying a catalyst ingredient on it is generally used.
By using this catalyst and proton conduction material to form a catalyst layer and forming networks of gas diffusion paths comprised of pores formed by the voids of the materials, proton conduction paths comprised of electrolytic materials, and electron conduction paths comprised of the carbon material, it becomes possible to realize the function of a so-called “gas electrode”.
In particular, for the proton conduction paths, a polymer electrolytic material comprised of a perfluorosulfonic acid polymer or a styrene divinyl benzene-sulfonic acid or other ion exchange resin is used. These generally used ion exchange resins exhibit a high proton conductivity only under a wet environment and end up dropping in proton conductivity under a dry environment.
This is believed to be because proton mobility requires the intervention or accompaniment of water molecules. Therefore, to enable a fuel cell to work efficiently, the electrolytic material has to constantly be in a wet state and water vapor has to be constantly supplied together with the gas supplied to the two electrodes.
In general, to supply water to the electrolytic material, the method is employed of wetting the gas supplied to the cell and operating the cell under the condensation point. According to this method, the water vapor supplied into the cell partially condenses and forms drops of condensed water.
Further, the above-mentioned cathode reaction produces water on the cathode catalyst. While depending on the operating conditions of the cell, the water produced as a result of the water vapor in the catalyst layer becoming supersaturated and condensing to form drops of condensed water.
The drops formed by the condensation of the water produced by these reactions or by the condensation of water vapor supplied for wetting in the catalyst layer block the gas diffusion paths. This phenomenon is called “flooding” and is remarkable in a cathode producing a large amount of water at the time of discharge of a large current and invites a sharp drop in the voltage.
In this way, to stably operate a fuel cell, it is necessary to satisfy the contradictory requirements of sufficiently wetting the inside of the catalyst layer and quickly discharging condensed water out from the system. For this purpose, in the past, it has been proposed to use PTFE or a silane coupling agent etc. to make the inside of the catalyst layer hydrophobic.
Japanese Patent Publication (A) No. 5-36418 proposes to introduce PTFE powder, Japanese Patent Publication (A) No. 4-264367 PTFE colloid, Japanese Patent Publication (A) No. 7-183035 carbon powder made hydrophobic by PTFE, and Japanese Patent Publication (A) No. 2000-243404 carbon material made hydrophobic by a silane coupling agent, into the catalyst layer so as to increase the hydrophobicity of the inside of the catalyst layer and enable condensed water to be quickly discharged out of the system.
Further, from the viewpoint of realizing practical fuel cells, reduction of cost becomes an important issue. In general, platinum is used for the electrode catalysts of both the anode and cathode of a solid polymer type fuel cell.
Platinum is the metal with the highest activity with respect to the oxygen reduction reaction and hydrogen oxidation reaction in a sulfonic acid-based proton conduction resin or other acidic electrolyte. Further, platinum is the most suitable catalyst material from the viewpoint of the stability as a metal.
Therefore, to reduce the cost, how far the amount of use of platinum can be reduced is an important issue.
As a method for improving the efficiency of utilization of the electrode catalysts and reducing the amount of use of platinum, the following specific technology for improvement has been proposed.
Japanese Patent Publication (A) No. 9-167622 describes a method of using carbon black having pores of a diameter of 8 nm or less accounting for 0.5 cm3/g or less of the volume as a carrier and carrying a precious metal on it to control the adsorption of catalyst metal particles at carrier pores where the polymer electrolyte serving as the path of movement of the protons cannot reach.
Further, Japanese Patent Publication (A) No. 2000-100448 describes using carbon black having pores of a diameter of 6 nm or less accounting for 20% or less of the overall pores.
As the method for improving the diffusion ability of reaction gas to an electrode catalyst surface, for example, Japanese Patent Publication (A) No. 2003-201417 describes using carbon black having a specific surface area as measured by the BET method of 250 to 400 m2/g, a particle size of 10 to 17 nm, and a total volume of pores opening to the surface and having a radius of 10 to 30 nm of 0.40 to 2.3 cm3/g as a catalyst carrier.
Japanese Patent Publication (A) No. 2004-82007 proposes improving the catalyst performance utilizing the relief shapes of a carrier surface.
That is, by using a carbon carrier selectively exposing the edges of a carbon net surface formed by a graphene sheet as the inside walls of pores to control the average pore size to 0.5 to 5.0 nm and carrying catalyst fine particles at the pore parts, the contact area between the carrier and the catalyst metal is increased. As a result, the catalyst activity of the catalyst metal itself is enhanced. Not only this, but also since the catalyst fine particles are carried at the pores, the so-called sintering phenomenon can be suppressed.
Further, the edge parts of the carbon net surface are preferably given —COOH, —OH, or other oxygen-containing functional groups, these oxygen functional groups result in stronger bonds between the Pt and other fine particles of catalyst metal and the carbon carrier and in improved catalyst activity.
While measures are being taken to reduce costs by the effective utilization of platinum as explained above, catalysts able to replace the basic cause of the high costs, that is, the platinum, are being energetically researched.
Among these, as a catalyst having an oxygen reduction ability, in the past, polyporphyrin, phthalocyanine, dibenzotetraazaannulene, and another complexes of large ring compounds containing a metal have been studied (H. Jahnke, M. Schonborn, G. Zimmermann, Topics in Current Chemistry, Vol. 61, p. 133 to 181 (1976)).
These metal complexes are known as mediators of oxygen in the body, that is, the basic idea is to utilize the ability to adsorb oxygen molecules for a reduction reaction on the electrochemical oxygen molecules (Yuasa, Makoto, Journal of the Japan Oil Chemistry Society, vol. 49, no. 4, p. 315 to 323 (2000)).
At the start of the research, the focus of the study was on the practical use for a catalyst for an oxygen electrode of a phosphoric acid type fuel cell, but problems remain such as the deterioration of the catalyst due to the phosphoric acid and the lower catalyst activity compared with platinum. Use for a phosphoric acid type fuel cell has still not been achieved.
On the other hand, in the case of a polymer solid electrolyte type fuel cell, it is believed that deterioration of the catalyst under an acidic environment can be avoided, so in recent years new energetic research has been under way.
To use these metal complexes as catalysts for practical electrodes, it is essential to immobilize the catalyst at an electron conductor. An oxygen carrier is used for this.
Specifically, carbon black having a high electron conductivity and a large surface area is used. Depending on the combination of this carbon carrier and metal complex, continuous use of the electrode catalyst becomes possible.
There are two problems with using a metal complex carried on a carbon carrier as an oxygen reduction catalyst: the overvoltage is larger than with a platinum catalyst and the reduction product is not only water (called a “four-electron reaction product”), but also a mixture with hydrogen peroxide (called a “two-electron reaction product”).
As a means for dealing with overvoltage, heat treatment in a nonoxidizing atmosphere has been proposed (J. A. R. van Veen, et al., J. Chem. Soc., Faraday Trans. 1, vol. 77, p. 2827 (1981)).
However, the overvoltage improved after the heat treatment is, compared with platinum, 0.1V or more. Problems continue to remain in terms of practical use.
Further, for improvement of the yield of four-electron reaction products, binuclear complexes (Japanese Patent Publication (A) No. 11-253811, F. C. Anson, et al., J. Am. Chem. Soc., Vol. 113, p. 9564 (1991)), dimerization of polyporphyrin complexes (J. P. Collman, et al., J. Am. Chem. Soc., Vol. 102, p. 6027 (1980)), etc. have been proposed. However, problems remain such as the difficulties in industrial use such as with the yield in synthesis, the high cost, and the large overvoltage compared with platinum or platinum alloy.
On the other hand, the gas diffusion layers positioned at the outsides of the catalyst layers of the two electrodes are required to have the function of uniformly dispersing the gas from the gas channels formed in separators to the catalyst layers and the function of conducting electrons between the catalyst layers and separators. Various techniques have been proposed up to now for enabling efficient operation of the fuel cells.
Among these, the technique of separating the gas diffusion layer into a two-layer structure has succeeded in realizing a certain performance.
Normally, the separator side has a first layer having relatively large pore sizes stressing the diffusion ability of the gas, while the catalyst layer side has a second layer made to function as an intermediate layer for securing electron conductivity and uniformity between the coarse structure first layer and the microstructure catalyst layer.
For example, U.S. Pat. No. 5,620,807 proposes a hydrophobic gas diffusion layer having carbon particles or fiber and a fluororesin as main ingredient. As a preferred embodiment, it proposes a two-layer structure having different porosities or pore sizes.
Further, Japanese Patent Publication (A) No. 10-261421 proposes as a structure of the gas diffusion layer a two-layer structure comprised of carbon fiber cloth on the surface of which is formed a layer comprised of a fluororesin and carbon black as main ingredients.
Further, Japanese Patent Publication (A) No. 2001-57215 proposes an intermediate layer having at least two pore size distribution centers between an electrode substrate and a catalyst layer. As a preferable embodiment, it proposes two or more types of carbon particles with different particle size distributions as main ingredients.